Thermoelectric based generators have been used successfully and reliably for the past 40 years to power deep space probes. These solid-state devices rely only on a temperature gradient to produce electricity, and are thus an attractive way of reducing our demand on fossil fuels. In order for thermoelectric devices to be used in large-scale applications, however, a number of materials problems must be overcome. For instance, the materials used in such devices are typically associated with low abundance, high cost, high toxicity, low thermal stability, and/or poor oxidation resistance.
Silicon-based thermoelectric materials are attractive candidates for large-scale applications since they are composed of abundant and low cost-elements and are relatively non-toxic. Such materials are typically associated with high thermal stability and oxidation resistance, in addition to electronic properties that are easily tunable (e.g., via doping). A problem with silicon-based thermoelectric materials, however, is that they generally have a thermal conductivity an order of magnitude too high to be an effective generator of thermoelectric power.
Earlier work investigated the impact on the lattice thermal conductivity of silicon-based materials by nanostructured inclusions. For instance, Fleurial and co-workers used a spark erosion technique to introduce small (e.g., 5 nm) inert inclusions of BN, B4C, and Y2O3 into fine grained Si0.80Ge0.20 (J.-P. Fleurial et al., Nanostructured Materials, 1995, 5(2), 207-223). Although the resulting composite demonstrated a 40% reduction in the lattice thermal conductivity with an overall increase in figure of merit (ZT) of about 20%, the use of spark erosion to form such composites is impractical for large-scale applications. In particular, spark erosion is too slow, costly, involves a complex set-up, and is low yielding.
Other methods have been investigated including the mixing of pre-fabricated or synthesized metal nanoparticles and Si powders, and then compacting them using hot pressing (S. Bux et al., Advanced Functional Materials, 2009, 19, 2445-2452). However, such methods are limited by several factors in that commercially available nanoparticles are larger than the particle size required by thermoelectric models, unavailable for specific compositions and materials, and too costly. Moreover, such nanoparticles are often functionalized or contaminated with oxide or thiolated capping agents which results in contamination of the resulting composite in addition to undesirable doping effects. Other problems that arise from this approach include the agglomeration of nanoparticles into larger particles to minimize surface energy, and the settling of nanoparticles at grain boundaries as opposed to within the matrix as required by composite thermoelectric models. Such characteristics in the final composite lead to undesirable thermoelectric transport properties.
More recent work investigated the formation of silicide inclusions in silicon-based materials. For instance, Mingo and co-workers theorized that a small volume fraction of silicide inclusions in a Si1-xGex matrix would result in an overall reduction in thermal conductivity of the silicon-germanium (N. Mingo et al., NanoLetters, 2009, 9(2), 711-715). In an approach to form such composites, Bux and co-workers used ball milling of Si0.80Ge0.20 and W (and Si and W) followed by hot pressing to form WSi2 inclusions in the Si matrix; however, due to the inclusions being too large and their size distribution too broad, the resulting composite did not demonstrate a significant reduction in lattice thermal conductivity (S. Bux et al., Materials Research Society, Symposium Proceedings, 2010, 1267, 1267-DD01-06). What is needed is a nanocomposite thermoelectric material having a lattice thermal conductivity suitable for thermoelectric applications, and a method of making such a material. Surprisingly, the present invention meets this and other needs.